Historically, wind turbines have been very small contributors to overall power generation to supply electrical grids. The low unit ratings (<100 kW) and the uncertain availability of wind sources causes wind turbine generators to have a negligible effect when power grid operators consider the security of the grid. However, wind turbine generators with ratings of 1.5 MW or more are now available. Furthermore, many power generation developers are installing wind farms having one hundred or more wind turbine generators. The “block” of power available from wind farms with 1.5 MW wind turbine generators is comparable to a modem gas turbine generator. Accordingly, wind turbine generators are increasingly feasible sources of power for the power grid.
In order to reliably supply power to the power grid, wind turbine generators (as well as other types of generators) must conform to power grid interconnection standards that define requirements imposed on power suppliers and large power consumers. In particular, a “low voltage ride through” (LVRT) requirement typically requires that a power generation unit must remain connected and synchronized to the grid when the voltage at the terminals of the generation unit fall to prescribed levels.
The LVRT requirement has been addressed in steam and gas turbine generators plants through use of vital electrical buses that are powered by DC power sources and by auxiliary buses connected to the generating units. These types of generation units are generally more resistant to voltage fluctuations than wind turbine generators.
In the past, wind turbine generators have been allowed to trip offline during a low voltage event. However, this does not satisfy LVRT requirements. Currently, wind turbine generators' specifications can require connection and synchronization with the power grid down to levels of 70% of rated voltage. These requirements can be accommodated through, for example, increased capacity in various components (motors, generators, converters, etc.). However, more severe voltage fluctuations, for example, voltages at 30% of rated voltage cannot be accommodated using these techniques.